1. Field of the Invention
The present invention relates to a pattern inspection apparatus, a pattern inspection method, or a program which causes a computer to execute the method, such as a pattern inspection technique which inspects a pattern defect of an object used in manufacturing a semiconductor, and an apparatus which inspects a defect of a considerably small pattern of a photomask, a wafer, or a liquid crystal substrate used in manufacturing a semiconductor device or a liquid crystal display (LCD).
2. Related Art
In recent years, with a high integration density and a large capacity of a large-scale integrated circuit (LSI), a circuit line width required for semiconductor devices has increasingly been narrowed. These semiconductor devices are manufactured such that a pattern is exposed and transferred on a wafer by a reduced projection exposure device called a stepper while using an original pattern with a circuit pattern formed thereupon (The original pattern is also called a mask or a reticle. The original pattern will be generally called as a mask hereinafter.) to form a circuit. Therefore, in manufacturing a mask to transfer the fine circuit pattern on a wafer, a pattern drawing device which can draw a fine circuit pattern is utilized.
An improvement in yield is essential in manufacturing an LSI which requires a lot of manufacturing cost. However, as represented by a 1-gigabit DRAM (Dynamic Random Access Memory), the order of a pattern constituting an LSI has been changing from a sub-micron order to a nano order. As one serious factor which decreases a yield, a pattern defect of a mask used when an ultrafine pattern is exposed and transferred on a semiconductor wafer by a photolithography technique is known. In recent years, with a miniaturization of an LSI pattern formed on a semiconductor wafer, a size which must be detected as a pattern defect has also become considerably small. For this reason, a pattern inspection apparatus which inspects a defect of a transfer mask used in manufacturing an LSI must be increased in precision.
On the other hand, with development of multimedia, an LCD (Liquid Crystal Display) has a liquid crystal substrate size of a 500 mm×600 mm or larger, and micropatterning of a TFT (Thin Film Transistor) or the like formed on a liquid crystal substrate advances. Therefore, it is required that a considerably small pattern defect be inspected in a large area. For this reason, development of a pattern inspection apparatus which efficiently inspects a defect of a pattern of a large-area LCD and a photomask used in manufacturing the large-area LCD in a short period of time is urgently required.
In this case, in a conventional pattern inspection apparatus, it is known that image data obtained by picking up an image of a pattern formed on a target plate such as a lithography mask or the like at a predetermined magnification by using a magnifying optical system is compared with design data or an image data obtained by picking up the image of the same pattern on the target plate to perform inspection.
For example, as pattern inspection methods, “die-to-die inspection” which compares image data obtained by picking up images of the same patterns at different places on the same mask and “die-to-database inspection” which generates design image data based on drawing data obtained by converting CAD data used in drawing a mask pattern into data in an inspection apparatus input format and compares the design image data with image data serving as measurement data obtained by picking up the image of a pattern are known. In the inspection methods in the inspection apparatus, a target plate is placed on a stage, and a flux of light scans the target plate as a result of the movement of the stage to perform inspection. The flux of light is irradiated on the target plate from a light source and an illumination optical system. Light transmitted through the target plate or reflected by the target plate is focused on a sensor through an optical system. The image picked by the sensor is transmitted to a comparing circuit as image data. In the comparing circuit, after alignment of the images, the image data, which has been picked up, is compared with the other image data or with design image data based on an appropriate algorithm. When the measurement data is different from the reference data, it is determined that a pattern defect is present.
An example of an algorithm of pattern inspection will be described below based on the case of “die-to-die inspection.”
When a target plate to be inspected is scanned, a sensor which scans a target plate observes a pattern in a region having a certain area. The region has a portion having a pattern and a portion having no pattern. As a result, from the sensor, grayscale data corresponding to a ratio of the pattern-present portion to the pattern-absent portion (occupation rate) is output in consideration of the optical characteristics or the like of the device. Two regions are compared with each other by comparing two image data output from the regions. When the data (occupation rates or grayscale values) are same with each other, it is determined that the patterns are same with each other. When the image data are different from each other, it is determined that the patterns are not same with each other. In general the image data is described as 8-bit pixel data and corresponds to 256 grayscales. For this reason, unless the image data are completely equal to each other, it is strict that a pattern defect is included in any one of both the patterns. However, in consideration of actual mask precision or the like, it is not practical that a pattern defect is included in any one of both the patterns. For this reason, in consideration of a difference between the comparison data, sameness or difference of the pattern is generally determined.
At this time, the degree of difference between the comparison data when the patterns are determined as the same patterns is called a threshold value. The threshold value is arbitrarily determined depending on various parameters of the inspecting apparatus.
In recent years, the line width of a design pattern has been narrowed, the presence of a micropattern for optical proximity correction (OPC) makes it difficult to match a design pattern and output data of measurement or output data of measurement when the images of same patterns at different positions on the same mask are picked, and objects which are not desired to be essentially determined as defects are occasionally seen as defects (pseudo defects). However, if the decision threshold value is moderated, a defect in a pattern which requires a high dimensional accuracy cannot be detected. For this reason, a drawn pattern is required to be compared and inspected at a plurality of inspection accuracies.
A technique which classifies drawn patterns in a plurality of ranks and compares and inspects the drawn patterns is disclosed in documents (for example, see Japanese Patent Application, Publication No. 2004-191957). However, in the patent document, a method of embodying a manner of classifying drawn patterns into a plurality of ranks in an apparatus is not concretely described. The technique is not sufficient in terms of usefulness, and a method of solving the problem is desired. Furthermore, a method of automatically detecting an OPC pattern from design image data (image data) to reduce pseudo defects in number is proposed (for example, see Japanese Patent No. 34131110).
When a defect appears in a target plate, a user generally reviews the defect. However, a considerably larger number of micropatterns and the like for the above optical proximity correction (OPC) are arranged (for example, at several ten thousands). For this reason, if all the micropatterns are determined as defects, quantity of work and time required to review the defects by a user exceed their limitations. As described above, when a large number of pseudo defects appear in the target plate, the inspection itself must be disadvantageously performed again. Alternatively, an expensive target plate itself must be disadvantageously manufactured again. In contrast to this, there is a problem that a defect in a pattern which requires a high dimensional accuracy cannot be detected if a decision threshold value is moderated.